Flexible and easy to use HSDP setting

[Liyang90]

What does this PR do?

Fixes #19502

It allows users to provide device_mesh as a tuple to the FSDPStrategy.

The test case for hybrid fsdp strategy would be broken, because _init_sharding_strategy is moved out of the __init__ function.

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📚 Documentation preview 📚: https://pytorch-lightning--19504.org.readthedocs.build/en/19504/

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 6 lines in your changes missing coverage. Please review.

Project coverage is 59%. Comparing base (1a6786d) to head (949d36f).

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #19504     +/-   ##
=========================================
- Coverage      84%      59%    -25%     
=========================================
  Files         426      421      -5     
  Lines       35264    35182     -82     
=========================================
- Hits        29552    20772   -8780     
- Misses       5712    14410   +8698     

[awaelchli]

Great job. Looking forward to this feature.

[npuichigo]

Any updates on this?

[Liyang90]

Hi @awaelchli , I just fixed a merge conflict with the master and pushed the change. Could you take another look? Let me know if you have any concerns.

[Liyang90]

All checks have passed. But I don't have access to merge the PR.

[awaelchli]

@Liyang90 Thank you for contributing this, and apologies for the delay.
I allowed myself to push a minor change regarding the import and extending the test.